Plating bath composition for electroless plating of gold and a method for depositing a gold layer

ABSTRACT

wherein the residues R1 and R2 comprise 2 to 12 carbon atoms and are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R1 and R2 are the same or different and a method of depositing of gold. The electroless aqueous gold plating bath is suitable to provide soft gold layers useful for wire bonding and soldering applications which are required for electronic components.

FIELD OF THE INVENTION

The present invention relates to electroless aqueous gold plating bath compositions for electroless plating of gold layers onto a substrate and a method for depositing gold. The plating bath is particularly suitable in the manufacture of printed circuit boards, IC substrates, semiconducting devices, interposers made of glass and the like.

BACKGROUND OF THE INVENTION

Gold layers are of paramount interest in the manufacturing of electronic components and in the semiconductor industry. Gold layers are frequently used as solderable and/or wire bondable surfaces in the manufacture of printed circuit boards, IC substrates, semiconducting devices and the like. Typically, they are used as a final finish before soldering and wire bonding. In order to provide electrical connections of sufficient conductivity and robustness between the copper lines and wires that are bonded thereto while providing a good strength for wire bonding, there are various layer assemblies which are used conventionally in the art. Among others, there are electroless nickel electroless gold (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG), direct immersion gold (DIG), electroless palladium immersion gold (EPIG) and electroless palladium autocatalytic gold (EPAG). Even though these techniques have been established some time ago, there are still many challenges unresolved. Such challenges are the corrosion of nickel layers which are placed between gold and copper lines (nickel corrosion) and insufficient stabilities of gold plating baths which is highly undesirable due to the cost of said baths. Also, it is highly desirable to deposit gold layers with sufficient plating rates to run manufacturing processes economically. Another desired property of the gold layer is the optical appearance that should be lemon yellow as a discolouration of the gold layer is not acceptable.

Due to the minuscule sizes of electrical components nowadays, it is not possible to use electrolytic processes which require electrical connections to the substrates. Therefore, electroless metal deposition processes (electroless plating) are used. Electroless plating generally describes methods without using external current sources for reduction of metal ions. Plating processes using external current sources are commonly described as electrolytic or galvanic plating methods. Non-metallic surfaces may be pretreated to make them receptive or catalytic for metal deposition. All or selected portions of a surface may suitably be pretreated. The main components of electroless metal baths are the metal salt, a reducing agent, and, as optional ingredients, a complexing agent, a pH adjuster, and additives, as for example stabilising agents. Complexing agents (also called chelating agents in the art) are used to chelate the metal being deposited and prevent the metal from being precipitated from solution (i.e. as the hydroxide and the like). Chelating metal renders the metal available to the reducing agent that converts the metal ions to metallic form.

A further form of metal deposition is immersion plating. Immersion plating is another deposition of metal using neither external current sources nor chemical reducing agents. The mechanism relies on the substitution of metals from an underlying substrate for metal ions present in the immersion plating solution. This is a distinct disadvantage of immersion plating because deposition of thicker layers is normally limited by the layer porosity.

In most cases, electroless gold plating baths use one or both types of electroless plating. Even if reducing agents have been added to the plating bath, immersion- type plating may occur albeit in a significantly reduced proportion.

In the context of the present invention electroless plating is to be understood (mainly) as autocatalytic deposition with the aid of a chemical reducing agent (referred to as “reducing agent” herein).

US 2012/0129005 A1 discloses an electroless gold plating bath comprising a water-soluble gold compound and an alkylene diamine, dialkylene triamines or the like. However, such gold plating solution lack sufficient stability and plating rate and are thus not applicable in industrial processes (see example 4).

US 2008/0138507 A1 reports electroless gold plating baths which use aldehyde compounds as reducing agents and N-substituted ethylene diamine derivatives such as N¹,N²-dimethylethylenediamine and N¹,N²-bis-(methylol)ethylenediamine. But again, the plating baths described therein lack plating rate and stability (see example 4). It is typically sufficient that gold plating bath have plating rates of 150 nm/h or more, preferably of 200 nm/h or more or ideally, of 250 nm/h or more to comply with today's industrial requirements.

OBJECTIVE OF THE INVENTION

It is the objective of the present invention to provide an aqueous electroless gold plating bath composition from which gold layers can be deposited with sufficient plating rate and a method for said purpose. It is another objective of the present invention to provide an electroless aqueous gold plating bath which has sufficient stability and which can be used for prolonged duration of time.

It is yet another objective that the gold layers, which are formed, show no discolouration.

SUMMARY OF THE INVENTION

These objectives are solved by the electroless aqueous gold plating bath according to the invention, which comprises at least one source of gold ions and at least one reducing agent for gold ions, and is characterized in that it comprises at least one ethylenediamine derivative as plating enhancer compound according to formula (I)

wherein the residues R¹ and R² comprise 2 to 12 carbon atoms and are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R¹ and R² are the same or different.

These objectives can further be solved by the method for depositing gold layers from the plating bath above and the use of the plating enhancer compound above in a gold plating bath, comprising at least one source of gold ions and at least one reducing agent for gold ions.

DESCRIPTION OF FIGURES

FIG. 1 shows a test substrate having a multitude of copper pads to be plated upon. Also depicted are 10 different spots where the layer thickness is measured (circles labelled 1 to 10).

DETAILED DESCRIPTION OF THE INVENTION

The ethylenediamine derivative according to formula (I) will be referred to herein as plating enhancer compound.

The plating enhancer compound according to formula (I)

bears the residues R¹ and R² which comprise 2 to 12 carbon atoms and are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R¹ and R² are the same or different.

The amine moieties in the plating enhancer compound of formula (I) are secondary amine moieties. It was found by the inventors that the respective diamine or a derivative thereof with methyl residues for R¹ and R² do neither allow for sufficient plating rates nor for sufficiently stable gold plating baths (see example 4).

In a preferred embodiment of the present invention the residues R¹ and R² of the plating enhancer compound of formula (I) comprise 2 to 8 carbon atoms, more preferred 2 to 6 carbon atoms, even more preferred 2 to 4 carbon atoms.

In another preferred embodiment of the present invention the residues R¹ and R² in formula (I) are the same. In yet another preferred embodiment of the present invention the alkyl residues R¹ and R² in formula (I) are free of terminal hydroxy moieties (—OH) as the inventors have found that terminal hydroxy moieties bound thereto are detrimental to stability of the plating bath (see example 4). In even another preferred embodiment of the present invention the residues R¹ and R² in formula (I) are free of terminal primary amino moieties as the inventors have found that said terminal amino moieties bound thereto are also detrimental to stability of the plating bath (see example 4). In another more preferred embodiment of the present invention, residues R¹ and R² are free of any further amino moieties and/or any hydroxy moieties. It is even more preferable that the alkyl residues are free of substituents and consist of carbon and hydrogen atoms only.

It is particularly preferred to select the plating enhancer compound from the following group consisting of N¹,N²-diethylethane-1,2-diamine, N¹,N²-dipropylethane-1,2-diamine, N¹,N²-di-iso-propylethane-1,2-diamine, N¹,N²-dibutylethane-1,2-diamine, N¹,N²-di-iso-butylethane-1,2-diamine, N¹,N²-di-tert-butylethane-1,2-diamine, N¹,N²-dipentylethane-1,2-diamine, N¹,N²-di-iso-pentylethane-1,2-diamine, N¹,N²-di-sec-pentylethane-1,2-diamine, N¹ ,N²-di-tert-pentylethane-1,2-diamine, N¹,N²-di-neo-pentylethane-1,2-diamine, N¹,N²-dihexylethane-1,2-diamine, N¹,N²-di-(1-methylpentyl)ethane-1,2-diamine, N¹,N²-di-(2-methylpentyl)ethane-1,2-diamine, N¹,N²-di-(3-methylpentyl)ethane-1, 2-diamine, N¹,N²-di-(4-methylpentyl)ethane-1,2-diamine, N¹,N²-di-(1,1-dimethylbutyl)ethane-1,2-diamine, N¹,N²-di-(1,2-dimethylbutyl)ethane-1,2-diamine, N¹,N²-di-(1,3-dimethylbutyl)ethane-1,2-diamine, N¹,N²-di-(2,2-dimethylbutyl)ethane-1,2-diamine, N¹,N²-di-(2,3-dimethylbutyl)ethane-1,2-diamine and N¹,N²-di-(3,3-dimethylbutyl)ethane-1,2-diamine.

It is most preferred that R¹ and R² are branched alkyl residues having 3 to 6 carbon atoms. It was surprisingly found that high plating rate in conjunction with even more improved bath stability are obtained when using branched alkyl residues having 3 to 6 carbon atoms for R¹ and R² (see example 5).

The concentration of the at least one plating enhancer compound according to formula (I) in the electroless aqueous gold plating bath according to the invention preferably ranges from 0.001-1 mol/L, more preferably from 10 to 100 mmol/L, even more preferably from 25 to 75 mmol/L. If more than one plating enhancer compound is contained in the electroless aqueous gold plating bath according to the invention the concentration is based on the total amount of substance of all plating enhancer compounds.

The electroless aqueous gold plating bath according to the invention is synonymously named an aqueous solution. The term “aqueous solution” means that the prevailing liquid medium, which is the solvent in the solution, is water. Further liquids, that are miscible with water, as for example alcohols and other polar organic liquids, that are miscible with water, may be added. In principle, an aqueous solution comprises more than 50 percent water by weight.

The electroless plating bath according to the invention may be prepared by dissolving all components in aqueous liquid medium, preferably in water.

The electroless aqueous gold plating bath according to the invention comprises at least one source of gold ions. Gold ions can be in either Au⁺, Au³⁺ or both oxidation states. The source of gold ions can be any water soluble gold salt having said oxidation states. Preferably, the source of gold ions is selected from the group consisting of gold cyanide, gold ammonium cyanide, gold (I) alkali cyanides including gold (I) potassium cyanide, gold (I) sodium cyanide, trisodium gold disulphite, tripotassium gold disulphite and triammonium gold disulphite, gold thiosulphate, gold thiocyanide, gold sulphate, gold chloride, and gold bromide. Preferably, the source of gold ions is a gold (I) alkali cyanide and may be added to the aqueous plating bath in the form of a solution containing this salt. The concentration of gold ions in the electroless aqueous gold plating bath according to the invention preferably ranges from 0.1 to 10 g/L, more preferably from 0.3 to 6 g/L.

The electroless aqueous gold plating bath further comprises at least one reducing agent for gold ions. The reducing agents for gold ions is preferably selected from the group consisting of aliphatic aldehydes such as formaldehyde, acetoaldehyde, propionaldehyde, n-butylaldehyde, α-methylvaleraldehyde, β-methylvaleraldehyde, γ-methylvaleraldehyde or the like; aliphatic dialdehydes such as glyoxal, succindialdehdye or the like; aliphatic unsaturated aldehydes such as croton aldehyde or the like; aromatic aldehydes such as benzaldehyde, o-nitrobenzaldehyde, m-nitrobenzaldehyde, p-nitrobenzaldehyde, o-tolylaldehyde, m-tolylaldehyde, p-tolylaldehyde, o-hydroxybenzaldehyde, m-hydroxybenzaldehyde, p-hydroxybenzaldehyde, phenylacetoaldehyde or the like; sugars having an aldehyde group (—CHO) such as glucose, galactose, mannose, ribose, maltose, lactose or the like; and precursors of formaldehyde such as urotropin, 1,3-bis(hydroxymethyl)-5,5-dimethylimidazolidine-2,4-dione (DMDM-hydantoin), paraformaldehyde, glyoxylic acid, sources of glyoxylic acid and glycolic acid. The term “source of glyoxylic acid” encompasses glyoxylic acid and all compounds that can be converted to glyoxylic acid in aqueous solution. In aqueous solution, the aldehyde containing acid is in equilibrium with its hydrate. A suitable source of glyoxylic acid is dihaloacetic acid, such as dichloroacetic acid, which will hydrolyse in an aqueous medium to the hydrate of glyoxylic acid. An alternative source of glyoxylic acid is the bisulphite adduct as is a hydrolysable ester or other acid derivative. The bisulphite adduct may be added to the electroless aqueous gold plating bath according to the invention or formed in situ. The bisulphite adduct may be made from glyoxylate and either bisulphite, sulphite or metabisulphite. Formaldehyde, sources of glyoxylic acid and glyoxylic acid are preferred, most preferred is formaldehyde.

The concentration of the at least one reducing agent for gold ions preferably ranges 0.0001 to 0.5 mol/L, more preferably 0.001 to 0.3 mol/L, even more preferably 0.005 to 0.12 mol/L.

Without being bound by theory, the inventors found with great inventive skill that reaction products of certain ethyleneamine derivatives such as triethylenetetraamine and reducing agents for gold ions such as formaldehyde (or its oxidised product formic acid) can be formed causing precipitation and reduced plating rates as a consequence. Typical reaction products are for example the respective aminal, enamine and amide derivatives. Therefore, it is preferable to limit the possible extent of undesired reaction product formation by choosing a molar ratio of plating enhancer compound according to formula (I) to reducing agent for gold ions in the electroless aqueous gold plating bath according to the invention to range from 0.5 to 9, preferably from 0.8 to 3.0, more preferably from 1.0 to 2.0 (see Example 6). If more than one plating enhancer compound according to formula (I) and/or more than one reducing agent for gold ions are used in the electroless aqueous gold plating bath according to the invention this ratio is calculated based on the total mass of substance of all respective individual compounds.

The electroless aqueous gold plating bath according to the invention optionally further comprises at least one complexing agent. The optional at least one complexing agent present in the electroless aqueous gold plating bath according to the invention is preferably selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, aminophosphonic acids or a salt of the aforementioned. The optional at least one complexing agent serves as a complexing agent for gold ions as well as for metal ions dissolved from the substrate during plating, e.g., nickel ions or copper ions. A preferred carboxylic acid is for example oxalic acid or a salt thereof. Preferred hydroxycarboxylic acids are for example tartaric acid, citric acid, lactic acid, malic acid, gluconic acid and salts of the aforementioned. Preferred aminocarboxylic acids are for example glycine, cysteine, methionine and salts of the aforementioned. Preferred aminophosphonic acids are nitrilotri(methylphosphonic acid) (commonly abbreviated as ATMP), diethylenetriaminepentakis(methylphosphonic acid) (commonly abbreviated as DTPMP) and ethylenediaminetetra(methylenphosphonic acid) (commonly abbreviated as EDTMP). In all cases also the sodium, potassium and ammonium salts of said compounds are also suitable. The concentration of the optional at least one complexing agent preferably ranges from 0.1 to 50 g/L, more preferably from 0.5 to 30 g/L.

More preferably, the electroless aqueous gold plating bath according to the invention comprises two different complexing agents and/or salts thereof, such as a hydroxycarboxylic acid or a salt thereof and an aminocarboxylic acid or a salt thereof.

The electroless aqueous gold plating bath according to the invention optionally comprises a crystal adjuster which is selected from the group consisting of thallium ions, arsenic ions, selenium ions and lead ions. Such crystal adjuster is preferably added to the electroless aqueous gold plating bath according to the invention in a concentration range of 0.00001 to 0.1 g/L. Useful sources for said ions can be water-soluble salts thereof such as the respective nitrates, sulphates and halides.

The electroless aqueous gold plating bath according to the invention optionally comprises at least one stabilising agent selected from the group consisting of sources of cyanide ions, hydantoin and alkyl derivatives thereof such as alkylhydantoin and dialkylhydantoin wherein alkyl residues in this context comprise C₁ to C₈ alkyls, preferably methyl, which can be cyclic and/or alicyclic, branched or unbranched, sulphur compounds such as 2-mercaptobenzothiazole, 2-mercaptobenzoimidazole, mercaptoacetic acid, 3-(2-benzthiazolylthio)-1-propanesulphonic acid, mercaptosuccinic acid, thiosulphuric acid, thioglycol, thiourea, thiomalic acid and the like, and aromatic nitrogen compounds such as benzotriazole,1,2,4-aminotriazole and the like. Suitable source of cyanide ions can be any soluble cyanide salts such as alkali cyanides including sodium cyanide, potassium cyanide.

The concentration of the optional stabilising agent can be selected dependant on its chemical structure and can be determined in routine experiments by anyone known in the art. The concentration of the optional stabilising agent preferably ranges 0.0000001 to 0.2 mol/L, it ranges more preferably from 0.000001 to 0.1 mol/L. Such stabilising agents are conventionally added to electroless gold plating baths to improve their lifetime and to prevent plate-out.

In a preferred embodiment, two or more stabilising agents are used. More preferably, a source of cyanide ions in a concentration of 0.0003 to 5 mmol/L and one or more of hydantoin and alkyl derivatives thereof in a concentration of 10 to 100 mmol/L and/or sulphur compounds in a concentration of 0.000001 to 0.05 mol/L is selected.

In another preferred embodiment of the present invention, the electroless aqueous gold plating bath according to the invention is free of intentionally added second sources of reducible metal ions (disregarding trace of impurities commonly present in technical raw materials) allowing for pure gold deposits to be formed. Pure gold deposits are soft, malleable, and particularly suitable for wire bonding and soldering. Traces of impurities are understood as compounds present in a technical raw material of 1 wt.-% or less.

The pH of the electroless aqueous gold plating bath according to the invention preferably ranges from 5 to 9, more preferably from 6 to 8, even more preferably from 6.5 to 7.5. The target pH value is adjusted by using for example acids such as phosphoric acid or bases such as sodium hydroxide or potassium hydroxide. It is advantageous and thus preferable to continuously control and adjust the pH value during plating as this also improves the plating bath lifetime.

The ethylenediamine derivative according to formula (I)

bearing the residues R¹ and R² which comprise 2 to 12 carbon atoms and which are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R¹ and R² are the same or different is used in aqueous gold plating baths to adjust the plating rate and improve their stability. Such aqueous gold plating baths can be electroless gold plating baths including immersion-type gold plating baths, autocatalytic gold plating baths and gold plating baths using a mixture of autocatalytic and immersion-type plating and electrolytic plating baths.

Preferably, the plating enhancer compound is used in electroless plating baths, preferably comprising at least one source of gold ions and at least one reducing agent for gold ions.

The method for depositing a gold layer onto a substrate, comprising, in this order, the steps

-   -   (i) providing a substrate;     -   (ii) contacting at least a portion of the surface of the         substrate with the aqueous gold plating baths according to the         invention described above;     -   and thereby depositing a gold layer onto at least a portion of         the surface of the substrate.

This contacting is preferably accomplished by dipping the substrate or the at least a portion of the surface of substrate into the plating bath or by spraying the plating bath onto the substrate or onto the at least a portion of surface of the substrate.

The at least a portion of the surface of the substrate preferably consists of a metal or metal alloy and gold is then deposited onto the at least a portion of the surface of the substrate consisting of a metal or metal alloy, selected from the group consisting of nickel, nickel alloys such as nickel phosphorous alloys, nickel boron alloys, cobalt, cobalt alloys such as cobalt phosphorous alloys, cobalt molybdenum phosphorous alloys, cobalt molybdenum boron alloys, cobalt molybdenum boron phosphorous alloys, cobalt tungsten phosphorous alloys, cobalt tungsten boron alloys, cobalt tungsten boron phosphorous alloys, palladium, palladium alloys such as palladium phosphorous alloys, palladium boron alloys, copper and copper alloys and gold or gold alloys. The electroless aqueous gold plating bath according to the invention can be used to deposit gold layers on gold substrates and may be utilised to thicken existing gold layers obtained for example from an immersion-type gold plating baths.

The substrates can be pretreated prior to plating, as it is known in the art. Such pretreatment includes cleaning steps with solvents and/or surfactants to remove mostly organic contaminants, etching steps with acids and optionally, oxidising or reducing agents to remove oxides and activation steps. The latter are to deposit a noble metal on the surface or a part thereof to make it more receptive for plating. Such noble metal can be palladium which can be deposited as a salt before it is reduced to elementary palladium on the surface. Or it can be deposited in a colloidal form and—where appropriate—be subjected to an acceleration step with an acid such as hydrochloric acid to remove any protective colloids such as tin colloids. Such an activation layer normally is not a discrete layer but an aggregation of island structures of palladium. However, activation layers are considered as metal substrates in the context of the present invention.

The temperature of the electroless aqueous gold plating bath according to the invention is preferably in the range of 30 to 95° C., more preferably from 70 to 90° C., even more preferably from 75 to 85° C., yet even more preferably from 77 to 84° C. during plating. The plating time is preferably in the range of 1 to 60 min, more preferably in the range of 5 to 30 min. However, if thinner or thicker deposits are desired, the plating time can be outside above-described ranges and adjusted accordingly.

It is preferable to continuously or in certain intervals replenish components which are being used during plating. Such components are inter alia the source of gold ions, the reducing agent for gold ions, the at least one stabilising agent and the plating enhancer compound. If necessary, the pH value can be adjusted continuously or in intervals as well.

The electroless aqueous gold plating bath according to the invention may be used with horizontal, vertical and spray plating equipment.

It is an advantage of the present invention that the stability of the electroless aqueous gold plating baths according to the invention is improved compared to gold plating baths known in the art (see example 5). Stability herein is to be understood as the lifetime of the bath prior to precipitation of compounds from the bath (“plate-out”) rendering it useless for plating purposes.

It is another advantage that the aqueous gold plating baths according to the invention allows for sufficient plating rates (deposited thickness of the plated metal layer over time) of 250 nm/h or more (see examples 1 to 3 and 5). Most plating baths known in the art which are somewhat stable do not allow for sufficient plating rates.

It is therefore a unique feature of the electroless aqueous gold plating bath according to the invention to provide highly stable gold plating baths with sufficient plating rates and thus allowing for more economically feasible gold plating processes.

The aqueous gold plating baths according to the invention form homogeneous gold deposits with little layer thickness diversion. The standard deviation of the gold layer thickness is below 10% or even below 8%. This little deviation is advantageously achievable even when plating on various substrates having different sizes.

The following non-limiting examples further illustrate the present invention.

EXAMPLES General Procedures

Pallabond® CLN, Pallabond® ME, PalleBond® Pre Dip, PallaBond® Aktivator and PallaBond® ACT V3 STD are products available from Atotech Deutschland GmbH. The source of gold ions was in all cases K[Au(CN)2].

Printed circuit test boards having a multitude of copper pads of different sizes ranging from 0.25 to 49 mm² on both sides were used in all experiments as substrates. They were cleaned and etched prior to activation with palladium. Then, palladium was deposited on the copper surfaces before the gold layer was plated thereon. The different pads where the layer thickness was determined are shown in FIG. 1. The individual pads had the following areas 1: 0.25 mm², 2: 0.52 mm², 3: 0.68 mm², 4: 0.97 mm², 5: 1.33 mm², 6: 1.35 mm², 7: 3.3 mm², 8: 6.7 mm², 9: 25 mm², 10: 49 mm².

Determination of Thickness of the Metal Deposits and Plating Rate

The deposit thickness was measured at 10 copper pads on each side of the test boards. The chosen copper pads had different sizes and are used to determine the layer thickness by XRF using the XRF instrument Fischerscope XDV-SDD (Helmut Fischer GmbH, Germany). By assuming a layered structure of the deposit the layer thickness can be calculated from such XRF data. The plating rate was calculated by dividing the obtained layer thickness by the time necessary to obtain said layer thickness.

The layer thickness homogeneity was determined as the standard deviation from the average thickness value.

Example 1 (Inventive): N¹, N²-di-iso-propylethane-1,2-diamine as Plating Enhancer Compound

A gold plating baths containing the following components was prepared by dissolution of all components in water:

potassium hydroxide to adjust pH to range from 7.9 to 8.1 plating enhancer compound   50 mmol/L complexing agent   89 mmol/L 5,5-dimethylhydantoin   47 mmol/L thallium ions 0.01 mmol/L potassium cyanide  0.6 mmol/L Formaldehyde 34.5 mmol/L gold ions  5.1 mmol/L

A substrate was subjected to the following process steps (Table 1) by dipping the substrates into the respective solutions employing the given parameters:

TABLE 1 Process sequence for gold plating. # Process step Solution T [° C.] t [min] 1 Cleaner Pallabond ® CLN 40 5 2 Rinse DI water 20 1 3 Etch Cleaner Pallabond ® ME 35 2 4 Rinse DI water 20 1 5 Pre Dip PallaBond ® Pre Dip 20 1 6 Activation PallaBond ® Aktivator 35 1 7 Rinse DI water 20 0.5 8 Rinse DI water 20 0.5 9 Electroless Pd PallaBond ® ACT V3 STD 45 3.5 10 Rinse DI water 20 2 11 Electroless Au Gold plating bath 82 20 12 Rinse DI water 20 2 13 Hot Rinse DI water (<2 μS/cm) 50 2 14 Drying — 60 15

After this process sequence the thicknesses of the individual metal layers were measured. The plating rate was calculated as described above.

TABLE 2 Layer thickness and plating rate for a gold plating bath containing N¹,N²-di-iso-propylethane-1,2-diamine. Layer thickness Standard deviation Plating rate Palladium layer  74 nm 444 nm/h Gold layer 103 nm 5.6% 309 nm/h

The gold layers were of lemon yellow colour. Also, the plating rate was very high and well above the desired minimum of 250 nm/h. The layer thickness distribution was very homogeneous, too, with only 5.6% deviation.

Example 2 (Inventive): N¹,N²-dipropylethane-1,2-diamine as Plating Enhancer Compound

The process as described in Example 1 was repeated wherein the gold plating bath contained 50 mmol/L N¹,N²-dipropylethane-1,2-diamine instead of 50 mmol/L N¹,N²-di-iso-propylethane-1,2-diamine. The results are summarized in the following table:

TABLE 3 Layer thickness and plating rate for a gold plating bath containing N¹,N²-dipropylethane-1,2-diamine. Layer thickness Standard deviation Plating rate Palladium layer  82 nm 492 nm/h Gold layer 101 nm 6.6% 303 nm/h

The gold layers were of lemon yellow colour. Also, the plating rate was very high and above the desired minimum of 250 nm/h. The layer thickness distribution was very homogeneous, too, with only 6.6% deviation.

Example 3 (Inventive): N¹,N²-diethylethane-1,2-diamine as Plating Enhancer Compound

The process as described in Example 1 was repeated wherein the gold plating bath contained N¹,N²-diethylethane-1,2-diamine instead of N¹,N²-di-iso-propylethane-1,2-diamine but in the same concentration. The results are summarized in the following table:

TABLE 4 Layer thickness and plating rate for a gold plating bath containing N¹,N²-diethylethane-1,2-diamine. Layer thickness Standard deviation Plating rate Palladium layer  86 nm 516 nm/h Gold layer 108 nm 6.4% 348 nm/h

The gold layers were of lemon yellow colour. Also, the plating rate was very high and clearly above the desired minimum of 250 nm/h. The layer thickness distribution was very homogeneous, too, with only 6.4% deviation.

Example 4 (Comparative): Use of Other Amines

The process as described in Example 1 was repeated wherein the gold plating bath contained other compounds as listed in Table 5 instead of N¹,N²-di-iso-propylethane-1,2-diamine. The results for 20 min gold plating are summarized in this table:

TABLE 5 Layer thickness and plating rate for comparative compounds in gold plating baths. Layer Plating thickness rate Complete Compound Concentration [nm] [nm/h] plate-out A.

5.61 g/L 6 18 — B.

5.11 g/L 10 30 — C.

5.86 g/L 20 60 — D.

5.26 g/L 28 84 — E.

3.00 g/L 7 21 — F.

4.40 g/L 50 150 ~3 d G.

7.40 g/L 200 600 <1 d H.

7.30 g/L 140 420 3 h

Various compounds containing amino moieties were tested. The stability tests were omitted in those cases where the respective plating rates were too low to meet today's industrial requirements of 250 nm/h plating rate.

Compound A comprised only tertiary amine moieties and did not bear any alkyl residues R¹ and R². Hardly any gold plating took place when using this compound as substitute for a plating enhancer compound in a gold plating bath. The gold layers were also very inhomogeneous and the standard deviation of the layer thickness was 58%.

Compound B was an alkylene diamine derivative comprising only primary and tertiary amino moieties (with only methyl residues). The gold plating was very slow when using this compound as substitute for a plating enhancer compound in a gold plating bath. The gold layers were also very inhomogeneous and the standard deviation of the layer thickness was 53%.

Compounds C and D are alkanolamine with a tertiary amino moiety only or with only one secondary amino moieties. The gold plating was slow when using these compounds as substitutes for a plating enhancer compound in a gold plating bath. The gold layers were also very inhomogeneous and the standard deviation of the layer thickness was 24% for compound C and 33% for compound D.

Compounds E and F did not contain any alkyl residues of sufficient length and when using these compounds as substitutes for a plating enhancer compound in a gold plating bath the plating was slow. Compounds E and F are of similar structure as the plating enhancer compound according to formula (I) but they either have no alkyl residues at all or the alkyl residues are short. In case of compound E, the gold layer thicknesses were inhomogeneous having a standard deviation of 14.4% while for compound F the deviation was 6.4%.

Compound G bore two terminal hydroxy moieties. When using this compound as substitute for a plating enhancer compound in a gold plating bath the plating rate was high but the stability of the gold plating bath was insufficient. Within less than 1 day the gold plating baths was irrevocably degraded and could not be used for gold plating anymore. The standard deviation of the gold layer thickness was 6.3%.

Compound H bore two terminal primary amino moieties. When using this compound as substitute for a plating enhancer compound in a gold plating bath the plating rate was sufficiently high but the stability of the gold plating bath was poor. Within 3 h the gold plating baths was irrevocably degraded. The standard deviation of the gold layer thicknesses was 8.5%.

In summary, comparative compounds A to F did not allow for sufficient plating rates of gold baths containing these compounds. The plating rates were always even below 200 nm/h and thus not sufficient for today's industrial demands.

Comparative compounds G and H as additives gave sufficient plating rates but the stability of the respective gold plating baths were unsatisfactory.

Example 5 (Inventive): Stability and Life-Time of Gold Plating Baths

The gold plating baths of examples 1 to 3 were used to deposit gold on substrates for a prolonged time. The stability of the gold plating baths and the plating rate were monitored over time. If a plate-out occurred the solution was filtered and re-used. Every day during the experiment, the pH value was measured and adjusted to 7.1 with KOH and/or H₃PO₄ if necessary. During plating, the source of gold ions, the source of cyanide ions and the plating enhancer compound were continuously replenished.

Table 6 provides information on the stability of gold plating baths containing different plating enhancer compounds. The plating baths were visually inspected directly after make-up (day 0) and for one week on a daily basis. The gold plating baths were also used to deposit gold on substrates every day during this test period. These results are summarized in Table 7. The values given in said table are the deposit thickness in nanometres obtained after 20 min of plating.

TABLE 6 Visual inspection of gold plating baths over time while in usage. N¹,N²- N¹,N²-di-iso- N¹,N²- diethylethane-1,2- propylethane-1,2- dipropylethane- Day diamine diamine 1,2-diamine 0 Slightly yellow and Colourless, clear Intensively yellow, clear solution solution clear solution 1 No changes No changes No changes 2 No changes No changes No changes 3 No changes No changes Slight plate-out 4 Slight plate-out No changes Slight plate-out 5 Slight plate-out No changes Slight plate-out 6 Slight plate-out No changes Slight plate-out 7 Slight plate-out No changes Slight plate-out

TABLE 7 Gold layer thickness after 20 min deposition from gold plating bath containing plating enhancer compounds over various days. N¹,N²- N¹,N²-di-iso- N¹,N²- diethylethane-1,2- propylethane-1,2- dipropylethane- Day diamine diamine 1,2-diamine 1 116 nm 105 nm 147 nm 2  91 nm  89 nm  91 nm 3 110 nm 106 nm 108 nm 4  99 nm  98 nm 106 nm 5 112 nm 112 nm 116 nm 6 108 nm 103 nm 101 nm 7  99 nm 101 nm 111 nm

Although in the cases of the linear plating enhancer compounds, N¹,N²-diethylethane-1,2-diamine and N¹,N²-dipropylethane-1,2-diamine slight precipitates occurred, the plating baths were still able to deposit gold layers without any plating rate reduction. The branched plating enhancer compounds, N¹,N²-di-iso-propylethane-1,2-diamine, showed no precipitates over 7 days and provided good plating rates over the entire tested period. It is thus deduced that plating enhancer compounds having branched alkyl residues result in improved bath stability.

Example 6 (Inventive): Ratio of Plating Enhancer Compound to Reducing Agent for Gold Ions

A gold plating baths containing the following components was prepared by dissolving all components in water:

Water 100 mL potassium hydroxide 12.4 g/L N¹,N²-di-iso-propylethane-1,2-diamine see table 8 complexing agent 89 mmol/L sulphur based stabilising agent 1.5 mg/L thallium ions 4.4 mg/L potassium cyanide  42 mg/L formaldehyde  0.3 g/L gold ion source 1.47 g/L

The gold plating bath was adjusted with KOH/H₃PO₄ to a pH value of 7.1. A substrate was subjected to the process as described in table 1 wherein the electroless gold plating step was carried out for 10 min only.

The process was repeated several times with different gold plating baths containing increasing amounts of plating enhancer compounds whereby the amount of reducing agent for gold ions was kept at the same level. The results are provided by table 8.

TABLE 8 Ratio of plating enhancer compound and reducing agent for gold ions. Molar ratio of N¹,N²-di-iso-propylethane- Plating rate 1,2-diamine to formaldehyde [%] 1/1 100 2/1 100 4/1 71.4 8/1 71.4 10/1  62.8

It can be seen that the highest plating rates can be obtained if the molar ratio of plating enhancer compound and reducing agent for gold ions ranges between 1 or 2 to 1. The plating rate dropped upon further increasing the amount of plating enhancer compound.

Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being defined by the following claims only. 

1. An electroless aqueous gold plating bath, comprising at least one source of gold ions and at least one reducing agent for gold ions, characterized in that the bath comprises at least one ethylenediamine derivative as plating enhancer compound according to formula (I)

wherein the residues R¹ and R² comprise 2 to 12 carbon atoms and are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R¹ and R² are the same or different.
 2. The electroless aqueous gold plating bath according to claim 1 characterised in that the residues R¹ and R² in formula (I) comprise 2 to 8 carbon atoms.
 3. The electroless aqueous gold plating bath according to claim 1 characterised in that the residues R¹ and R² in formula (I) are the same.
 4. The electroless aqueous gold plating bath according to claim 1 characterised in that the residues R¹ and R² in formula (I) are free of any further amino moieties and/or any hydroxy moieties.
 5. The electroless aqueous gold plating bath according to claim 1 characterised in that the residues R¹ and R² in formula (I) are branched alkyl residues having 3 to 6 carbon atoms.
 6. The electroless aqueous gold plating bath according to claim 1 characterised in that the concentration of the at least one plating enhancer compound according to formula (I) ranges from 0.001-1 mol/L.
 7. The electroless aqueous gold plating bath according to claim 6 characterised in that the concentration of the at least one plating enhancer compound according to formula (I) ranges from 10 to 100 mmol/L.
 8. The electroless aqueous gold plating bath according to claim 1 characterised in that the at least one reducing agent for gold ions is selected from the group consisting of aliphatic aldehydes, aliphatic dialdehydes, aliphatic unsaturated aldehyde, aromatic aldehydes, sugars having an aldehyde group and precursors of formaldehyde.
 9. The electroless aqueous gold plating bath according to claim 1 characterised in that the molar ratio of reducing agent to plating enhancer compound according to formula (I) ranges from 0.8 to
 3. 10. The electroless aqueous gold plating bath according to claim 1 characterised in that the pH of the electroless aqueous gold plating bath ranges from 5 to
 9. 11. The electroless aqueous gold plating bath according to claim 1 characterised in that the concentration of gold ions ranges from 0.1 to 10 g/L.
 12. The electroless aqueous gold plating bath according to claim 1 characterised in that the electroless aqueous gold plating bath further comprises at least one complexing agent selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, aminophosphonic acids or a salt of the aforementioned.
 13. A method for depositing a gold layer onto a substrate, comprising, in this order, the steps (i) providing a substrate (ii) contacting at least a portion of the surface of the substrate with the electroless aqueous gold plating bath according claim 1 and thereby depositing a gold layer onto the at least a portion of the surface of the substrate.
 14. The method for depositing a gold layer onto a substrate according to claim 13 wherein the at least a portion of the surface consists of a metal or metal alloy and the metal or metal alloy is one or more selected from the group consisting of nickel, nickel alloys such as nickel phosphorous alloys, nickel boron alloys, cobalt, cobalt alloys such as cobalt phosphorous alloys, cobalt molybdenum phosphorous alloys, cobalt molybdenum boron alloys, cobalt molybdenum boron phosphorous alloys, cobalt tungsten phosphorous alloys, cobalt tungsten boron alloys, cobalt tungsten boron phosphorous alloys, palladium, palladium alloys such as palladium phosphorous alloys, palladium boron alloys, copper and copper alloys and gold or gold alloys.
 15. (canceled) 